Americans in London took the news largely in their stride. "My initial reaction was to be a bit concerned, but, in New York, terror alerts happen regularly, so we're used to it," said Bergen Swanson, 38, a film-maker visiting from the Big Apple.

A bubbly trio of women from Michigan said that seeing the news on a pub TV the night before had not put them off enjoying the remainder of a fortnight's visit.

Housewife Cynthia Miller, 50, said the warning had been distinctly vague, "to stay away from public places" and mentioned no particular European country. "They [the US authorities] said they weren't discouraging us from travelling," said Cherie McCoy, a massage therapist. "But we knew our husbands would be more worried, so we called them. It was more about the family than us."

Overall, though, they were more concerned about the weather. And if anything serious did happen, joked McCoy, "we could just pretend to be Canadian".

In Trafalgar Square, Alex Yabroff, 27, from Washington DC, was in London for work, and said he had not paid much attention. "I received a state department email about it. Something similar happened when I was a student after 9/11." He said he was leaving as planned on Tuesday. "Now's the time to get out right?" he said.